Switched-capacitor band-pass filter of a discrete-time type, in particular for cancelling offset and low-frequency noise of switched-capacitor stages

Abstract

A band-pass filter made up by an operational amplifier and by an input circuit. The input circuit is formed by a capacitive filtering element, connected to the input of the operational amplifier; a coupling switch, coupled between an input node and the capacitive filtering element; a capacitive sampling element, coupled between the input of the filter and the input node; and a sampling switch, coupled between the input node and a reference-potential line. The coupling switch and the input sampling switch close in phase opposition according to a succession of undesired components sampling and sensing steps, so that the capacitive sampling element forms a sampler for sampling the undesired component in the undesired components sampling step, in the absence of the component of interest, and forms a subtractor of the undesired components from the input signal in the sensing step.

The invention claimed is: 
     
       1. A switched-capacitor bandpass filter, comprising:
 a first input configured to receive a time-discrete signal formed by a first component and by an undesired component; 
 a first output; 
 an operational amplifier having a first input and a first output, the first output of the operational amplifier being coupled to the first output of the filter; 
 a first input branch including:
 an input node; 
 a filtering capacitive element having a first terminal coupled to the first input of the operational amplifier and a second terminal; 
 a coupling switch coupled between the input node and the second terminal of the filtering capacitive element; 
 an input sampling capacitive element coupled between the first input of the filter and the input node; and 
 an input sampling switch coupled between the input node and a reference potential line, 
 the coupling switch and the input sampling switch being configured so as to close in counterphase according to a sequence of undesired component sampling steps and sensing steps, so that the input sampling capacitive element forms a sampler of the undesired components in the undesired component sampling steps, in absence of the first component, and forms a subtractor of the undesired component from the time-discrete signal in the sensing step. 
 
 
     
     
       2. A filter according to  claim 1 , comprising:
 a first feedback capacitive element coupled between the first input and the first output of the operational amplifier; 
 a second feedback capacitive element coupled in parallel to the first feedback capacitive element; and 
 a first decoupling switch connected so as to decouple from each other the first and second feedback capacitive elements in the sensing step. 
 
     
     
       3. A filter according to  claim 2 , wherein the first decoupling switch is arranged between a first terminal of the first feedback capacitive element and a first terminal of the second feedback capacitive element. 
     
     
       4. A filter according to  claim 2 , wherein the first decoupling switch is arranged between the first feedback capacitive element and the first input or the first output of the operational amplifier. 
     
     
       5. A filter according to  claim 1 , further comprising a first correlated double-sampling circuit including:
 an output sampling capacitive element coupled between the first output of the operational amplifier and the first output of the filter; 
 an output sampling switch arranged between a first terminal of the output sampling capacitive element and the reference potential line; and 
 an output reset switch coupled to a second terminal of the output sampling capacitive element, 
 the output sampling switch being configured so as to close during the undesired component sampling step and the reset switch being configured so as to close in a reset step at the beginning of the undesired component sampling step. 
 
     
     
       6. A filter according to  claim 5 , wherein the operational amplifier is a fully differential amplifier and comprises a second input, a second output, and a second correlated double-sampling circuit being connected to the second output of the operational amplifier, the output reset switch being coupled between the first and second outputs of the operational amplifier. 
     
     
       7. A filter according to  claim 5 , further comprising an input reset switch coupled between the first input of the filter and the reference potential line and configured so as to close in the reset step. 
     
     
       8. A filter according to  claim 7 , wherein the operational amplifier is a fully differential amplifier and comprises a second input and a second output, the filter further comprising:
 a second input branch coupled between the second input of the filter and the second input of the operational amplifier; and 
 a further reset switch being coupled between the first and second inputs of the filter. 
 
     
     
       9. A filter according to  claim 1 , wherein the operational amplifier is a fully differential amplifier. 
     
     
       10. A time-discrete switched capacitor circuit, comprising:
 a switched capacitor discrete amplifier having an output; and 
 a filter that includes:
 a first input coupled to the output of the discrete amplifier and configured to receive a time-discrete signal formed by a first component and by an undesired component; 
 a first output; 
 an operational amplifier having a first input and a first output, the first output of the operational amplifier being coupled to the first output of the filter; 
 a first input branch including:
 an input node; 
 a filtering capacitive element having a first terminal coupled to the first input of the operational amplifier and a second terminal; 
 a coupling switch coupled between the input node and the second terminal of the filtering capacitive element; 
 an input sampling capacitive element coupled between the first input of the filter and the input node; and 
 an input sampling switch coupled between the input node and a reference potential line, 
 the coupling switch and the input sampling switch being configured so as to close in counterphase according to a sequence of undesired component sampling steps and sensing steps, so that the input sampling capacitive element forms a sampler of the undesired components in the undesired component sampling steps, in absence of the first component, and forms a subtractor of the undesired component from the time-discrete signal in the sensing step. 
 
 
 
     
     
       11. A circuit according to  claim 10 , wherein the discrete amplifier is a charge amplifier. 
     
     
       12. A circuit according to  claim 10 , wherein the filter includes:
 a first feedback capacitive element coupled between the first input and the first output of the operational amplifier; 
 a second feedback capacitive element coupled in parallel to the first feedback capacitive element; and 
 a first decoupling switch connected so as to decouple from each other the first and second feedback capacitive elements in the sensing step. 
 
     
     
       13. A circuit according to  claim 12 , wherein the first decoupling switch is arranged between a terminal of the first feedback capacitive element and a terminal of the second feedback capacitive element. 
     
     
       14. A circuit according to  claim 12 , wherein the first decoupling switch is arranged between the first feedback capacitive element and the first input or the first output of the operational amplifier. 
     
     
       15. A circuit according to  claim 10 , wherein the filter includes a first correlated double-sampling circuit including:
 an output sampling capacitive element coupled between the first output of the operational amplifier and the first output of the filter; 
 an output sampling switch arranged between a first terminal of the output sampling capacitive element and the reference potential line; and 
 an output reset switch coupled to a second terminal of the output sampling capacitive element, 
 the output sampling switch being configured so as to close during the undesired component sampling step and the reset switch being configured so as to close in a reset step at the beginning of the undesired component sampling step. 
 
     
     
       16. A circuit according to  claim 15 , wherein the operational amplifier is a fully differential amplifier and comprises a second input, a second output, and a second correlated double-sampling circuit being connected to the second output of the operational amplifier, the output reset switch being coupled between the first and second outputs of the operational amplifier. 
     
     
       17. A circuit according to  claim 15 , wherein the filter includes an input reset switch coupled between the first input of the filter and the reference potential line and configured so as to close in the reset step. 
     
     
       18. A circuit according to  claim 17 , wherein the operational amplifier is a fully differential amplifier and comprises a second input and a second output, the filter further comprising:
 a second input branch coupled between the second input of the filter and the second input of the operational amplifier; and 
 a further reset switch being coupled between the first and second inputs of the filter. 
 
     
     
       19. A method for reducing an offset and the noise of a switched capacitor filter that includes an input, an output, an operational amplifier, and a filtering capacitive element, the method comprising:
 receiving an input time-discrete two-phase signal, formed in a first phase by an undesired component and in a second phase by a first component and the undesired component; 
 sampling the time-discrete signal by decoupling the filtering capacitive element from the input of the filter; and storing the undesired component in an input sampling capacitive element that is selectively connected between the input of the filter and a reference potential line; and 
 subtracting the undesired component from the time-discrete signal by connecting the input sampling capacitive element between the input of the filter and the filtering capacitive element; and 
 generating, using the operational amplifier, a filtered first component at the output of the filter. 
 
     
     
       20. A method according to  claim 19 , further comprising resetting the filter at the beginning of the sampling by decoupling the input sampling capacitive element from the input of the filter and discharging the input sampling capacitive element. 
     
     
       21. A method according to  claim 20 , wherein:
 during the resetting, decoupling an output sampling capacitor, arranged between the output of the operational amplifier and the output of the filter from the operational amplifier and discharging the output sampling capacitor toward the reference potential line, 
 during the sampling, coupling the output sampling capacitor between the output of the operational amplifier and the reference potential line and storing in the output sampling capacitor undesired output components generated by the operational amplifier, and 
 during the subtracting, coupling the output sampling capacitor between the output of the operational amplifier and the output of the filter and subtracting, from the signal at the output of the operational amplifier, the stored undesired output components. 
 
     
     
       22. A method according to  claim 21 , further comprising coupling a feedback capacitive element between the input and the output of the operational amplifier during the subtracting and, during the resetting, decoupling the feedback capacitive element from the output of the operational amplifier.